Precision machine vision inspection systems (or “vision systems” for short) can be used to obtain precise dimensional measurements of inspected objects and to inspect various other object characteristics. Such systems may include a computer, a camera and microscope-type optical system, and a precision stage that is movable in multiple directions so as to allow the camera to scan the features of a workpiece that is being inspected. One exemplary prior art system that is commercially available is the QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, which is hereby incorporated by reference in its entirety. Such systems are known to incorporate various types of focus measurement, for governing autofocus and/or surface height measurements. One known type of focus measurement is based on analysis of the contrast in acquired images. For a given field of view, the highest contrast image generally corresponds to the best focused image. A surface height measurement may be inferred from the best focused image position, since the camera-object distance corresponding to any image is generally known in precision machine vision inspection systems.
Another type of focus and/or measurement is based on the use of an auxiliary focus sensor, which is a focus sensor that does not rely on the images of the machine vision inspection system for determining the best focus position or surface height. Various known types of auxiliary focus sensors have been used including triangulation sensors, knife edge focus sensors, chromatic confocal sensors, and the like. However, such known auxiliary sensors have exhibited deficiencies such as inadequate range vs. resolution capability, and/or inadequate robustness when tracking over abrupt steps in surface height.
One type of sensor for measuring changes of distance to a workpiece surface from an objective lens is described in U.S. Pat. No. 4,336,997, to Röss et al., which is hereby incorporated by reference in its entirety. The '997 patent discloses a configuration in which an objective lens may be focused on a measurement object, and a focus detector (e.g., a modal aperture stop located in front of a photoelectric converter, at a detector focus plane) may indicate deviations of the measurement object from the plane of best focus. However, the '997 patent does not disclose a focus detector with an unconventional range vs. resolution capability.
To obtain high resolution measurements of surface shape, a Shack-Hartmann type of wavefront sensing technique has been used. U.S. Pat. No. 6,184,974, to Neal et al., which is hereby incorporated by reference in its entirety, discloses that minute deviations of a surface from perfect flatness, such as the surface of a silicon wafer, etc., may be measured by reflecting appropriate illumination from the surface and directing it to a Shack-Hartmann wavefront sensor that includes a plurality of sub-apertures. However, the plurality of sub-apertures sense a relative surface profile, they do not sense an overall range (distance) to a surface, and the '974 patent does not disclose a detector configuration with an unconventional range vs. resolution capability.
U.S. Pat. No. 4,950,878, to Ulich et al., which is hereby incorporated by reference in its entirety, discloses an adaptive optics wavefront control system including a Shack-Hartmann type of wavefront sensing technique called a coarse/fine gradient sensor, comprising two Schack-Hartmann type sensors having different focal lengths and a different ranges and sensitivities. The configuration of the '878 patent provides an unconventional range vs. resolution capability. However, while the configuration of the '878 patent is suited for adaptive optics control, it is not well suited to the physical design constraints and the range requirements of a precision machine vision inspection system of the type outlined above.
A focus and/or range sensor that overcomes the foregoing and other disadvantages, would be desirable.